1. Field of the Invention
The present invention relates to desulfurization of fuels, and more specifically to optimized sorbent materials and processing for efficient desulfurization of high sulfur content fuels.
2. State of the Prior Art
Fuel cells powered by liquid hydrocarbon fuels promise to have very high power density and efficiency, which is of great interest in military and commercial markets. However, many conventional hydrocarbon fuels have high sulfur or sulfur compound contents usually in the form of organo-sulfur compounds, such as thiophenes and dibenzothiophenes, and such sulfur poisons the catalysts that are central to the conversion of fuel to electric energy in fuel cells. Therefore, for fuel cells to be usable with conventional fuels, the sulfur containing molecular species must be removed. This problem has been a detriment to development of fuel cell electric power generator systems, especially for small scale portable and mobile systems that would be used in circumstances that are not conducive to the use of large, fixed beds or other complex desulfurization systems, yet are likely to encounter fuels with too much sulfur for sustained fuel cell operation.
State of the art desulfurization systems utilize fixed beds of sorbent to selectively remove sulfur from fuels. When hydrocarbon fuels that contain sulfur compounds are flowed through the fixed beds of sorbent materials, the sulfur compounds are retained by the sorbent materials, while the hydrocarbon fuels exit substantially free of sulfur. When the sorbent materials become saturated with sulfur and other adsorbed materials and are no longer effective for further sulfur removal, the bed must be replaced. This state of the art has been inimical to the use of fuel cells to generate power from conventional fuels on portable platforms, such as automobiles, recreational vehicles, portable generators for industrial or military uses, or even ships. To be useful and practical, enough fuel must be desulfurized on the portable platform to accomplish the mission or to continue operating the fuel cell power generator until the next maintenance period. Therefore, to reduce the maintenance burden and still meet operational requirements, a large enough sorbent bed must be carried on the portable platform to treat enough fuel to keep the fuel cell operating for the duration of the maintenance interval. Of course, larger sorbent beds with more sorbent can desulfurize more fuel, but for most applications, the amounts of sorbent needed to provide enough desulfurized fuel for practical applications would be impractical to carry along on the portable platform. In addition, there would also be the need to have replacement sorbent available as well as the problem and expense of disposal of used sorbent.
Consequently, most of the research efforts to solve this problem have been directed toward finding or developing sorbent materials that are both selective, i.e., that minimize adsorption of non-sulfur species and have more available capacity for adsorption of sulfur species, and toward finding or developing sorbent materials that have more adsorption capacity, in general. The theory of that approach is that with more adsorption capacity and not wasting it on non-sulfur species, less sorbent would be needed to provide the fuel needs of any particular application. Such efforts to date have not been successful enough to make fuel cells practical for mobile power generation with conventional fuels, and there appears to be little likelihood of achieving such success in the near future.